1. Field of the Invention
The present invention relates to an information recording/reproduction apparatus for reproducing information as an (N+1)-value reproduction data train, and extracting a reproduction clock from the reproduction data train and, more particularly, to an information recording/reproduction apparatus for generating a reproduction signal by equalizing a partial response waveform.
2. Related Background Art
Conventionally, when information is recorded in an information recording/reproduction apparatus, input information is code-converted into a recording data train by a predetermined modulation method, and the recording data train is recorded as a recording signal on a recording medium. When recorded information is reproduced, a reproduction signal is compared with one slice level V.sub.0 to discriminate data "1" or "0". The reproduction process will be described below with reference to FIG. 1. In the following description, for the sake of simplicity, an original data train to be recorded is called input information, a data train which is converted according to a predetermined code-conversion rule, and is to be input to a precoder or an NRZI converter, is called an input data train, NRZI-converted data to be actually recorded on a recording medium is called a recording data train, a signal which is reproduced from the recording medium, and is subjected to waveform equalization is called a reproduction signal, and data obtained by binarizing the reproduction signal and detected as binary data by a reproduction clock is called reproduction data.
(a) of FIG. 1 shows a recording data train. A recording data waveform shown in (b) of FIG. 1 is generated according to the recording data train, and is recorded as a recording signal on a recording medium. When the recorded information is reproduced, a reproduction signal waveform shown in (c) of FIG. 1 is obtained. When the reproduction signal waveform is binarized by comparing it with the slice level V.sub.0, a reproduction data waveform shown in (d) of FIG. 1 is generated. (e) of FIG. 1 shows a reproduction clock, and a reproduction data train shown in (f) of FIG. 1 is detected using the reproduction clock. The reproduction clock is extracted under the PLL control using the reproduction data. Therefore, unless data bits "0" continuously appear in the recording data train, an edge is generated in the reproduction data, and the clock can be extracted from the reproduction data itself.
Recently, a high density and high transfer rate are required for an information recording/reproduction apparatus. However, when a high density and high transfer rate are realized in the information recording/reproduction apparatus, inter-code interference cannot be ignored due to narrowing of the detection window width, and the apparatus becomes complicated if the interference is to be removed, resulting in an increase in cost. As a means for solving this problem, recently, a means adopting partial response has been proposed. Partial response is a technique for reproducing information without removing inter-code interference by defining inter-code interference caused by transfer characteristics of a recording/reproduction system of the information recording/reproduction apparatus to be a convolution of information.
A reproduction system of an information recording/reproduction apparatus which adopts class 1 partial response (PR(1, 1)) will be described below with reference to the accompanying drawings. FIG. 2 shows signal waveforms of the reproduction system for an isolated bit. (a) of FIG. 2 shows a recording data train, and (b) of FIG. 2 shows a recording data waveform which is recorded on a recording medium. The recorded information is reproduced, and waveform equalization is performed, so that a reproduction signal has a reproduction signal waveform shown in (c) of FIG. 2. The bit interval of transfer data is T.sub.b, and a reproduction signal for two bits becomes 1 with respect to 1-bit data "1" when viewed from the reproduction signal. More specifically, data "1" has 1 as an inter-code interference in the next bit.
FIG. 3 shows reproduction signal waveforms obtained when arbitrary data is recorded upon execution of such waveform equalization. (a) of FIG. 3 shows input data. When the input data is precoded to prevent error transmission, a recording data train shown in (b) of FIG. 3 is obtained. (c) of FIG. 3 shows a recording data waveform of this recording data train, which waveform corresponds to a signal recorded on a recording medium. When the recorded signal is reproduced, and is subjected to the above-mentioned waveform equalization, a reproduction signal shown in (d) of FIG. 3 is obtained. This reproduction signal is ternary-detected by level-slicing this signal by two slice levels V.sub.1 and V.sub.2 shown in (d) of FIG. 3, thereby generating a ternary data train, as shown in (f) of FIG. 3. As a result of ternary detection, when data "1" is determined in correspondence with level 1, and data "0" is determined in correspondence with level 0 or 2, a reproduction data train can be generated, as shown in (g) of FIG. 3. Note that (e) of FIG. 3 shows a reproduction clock.
In the case of coding without partial response, PLL control is executed using a reproduction data signal obtained by comparing the reproduction signal with the slice level V.sub.0, as described above, thereby generating reproduction data synchronous with a reproduction clock. Therefore, the time interval required for the reproduction signal to cross the slice level, i.e., a change point of recording data from "1" to "0" or vice versa, e.g., as for NRZI codes, how many "0" s are present between two "1"s before NRZI conversion, determines the performance of codes in terms of controllability of PLL.
However, when the above-mentioned partial response is used, if "1"s continuously appear in input data (that is, a long string consisting entirely of "1"s with no intervening "0"s), then the level of the reproduction signal is kept at level 1, the reproduction signal crosses neither the slice level V.sub.1 nor V.sub.2. This state will be described below with reference to FIG. 4. (a) of FIG. 4 shows input data. When such data including continuous "1"s is to be recorded, a precoded recording data waveform shown in (c) of FIG. 4 is obtained. When this data is recorded, is reproduced, and is then subjected to waveform equalization, a reproduction signal waveform becomes as shown in (d) of FIG. 4. As can be seen from (d) of FIG. 4, the reproduction signal waveform crosses neither the slice level V.sub.1 nor V.sub.2 as long as "1"s continuously appear in the data. For this reason, in a self-clock system for extracting a reproduction clock from an edge of data, a reproduction clock cannot be extracted.
This problem will be discussed in more detail below. In this case, DC-free 8-9 NRZI codes will be exemplified. In the case of 8-9 NRZI codes, input information is divided into 8-bit blocks X.sub.i. For example, assume that Xi is 01001110. A flag bit "0" or "1" is added to the beginning of each block to generate input data Y.sub.i as a 9-bit data block. In this case, the data Y.sub.i is 001001110 or 101001110. Furthermore, the data Y.sub.i is NRZI-converted into a recording data train Z.sub.i. In this case, this data conversion is performed under an assumption that the bit at the end of the immediately preceding Z.sub.i is "0". Z.sub.i is 001110100 or 110001011.
As can be understood from this example, the recording data train Z.sub.i becomes bit-reversed patterns depending on the flag bit "0" or "1" and depending on this conversion result, DSV (Digital Sum Variation: a difference between the total number of data bits "0" and the total number of data bits "1" of the recording data train) of the NRZI-converted recording data bit train is calculated. The flag bit "0" or "1" is then determined, so that the DSV approaches 0. When input information is converted by this conversion rule, and the converted information is recorded, a conventional reproduction detection means can extract a reproduction clock from reproduction data itself since data bits "0" do not continuously appear in recording data. However, when the partial response is used, if input data includes continuous "1"s, since a recording data train also includes continuous "1"s, it is difficult to extract a reproduction clock from a reproduction signal for the above-mentioned reasons.